An electrical contact is described in U.S. Pat. No. 5,892,424 and represents an encapsulated point of contact of an electrical connection. The known electrical contact is made up of a substrate on which a contact layer is deposited to reinforce the wear resistance of the electrical contact. This contact layer has a matrix formed from a first element, which is doped by a second element. The matrix can be made up of an element selected from the group that includes Mo, Zr, Nb, Hf, Ta and W. The additional element may be an element selected from a group that includes Zn, Cd, Hg, Al, Ga, In, Tl, Ge, Sn, Pb, As, Sb and Bi. The additional element stabilizes the contact resistance of the electrical contact during an electrical switching operation. Furthermore, the additional element leads to an improvement in the wear resistance and the oxidation resistance of the electrical contact. The proportion of additional elements in the contact layer may amount to between 0.5 atom % and 50 atom %.
The contact layer in the known electrical contact is applied according to a sputtering method, an ion-supported vapor-deposition method, an ion-plating method or a plasma CVD method. However, due to a required ultra-high vacuum, these methods are involved and not suited for the production of high quantities.
Furthermore, the metals from which the contact layer of the known electrical contact is made, are expensive and therefore also not suitable for contacts that are required in large numbers. This applies especially to electrical contacts in motor vehicles, which are required in quantities of 1000 to 3000 pieces per motor vehicle.
In practice, electrical contacts in the automotive sector often have a contact layer made of tin. This layer may be a hot-dipped or a galvanically deposited layer having a thickness of a few micrometers. Tin is characterized by its ductility as well as its excellent electrical conductivity. When using a tin contact layer, diffusion causes an intermediate layer to be formed at the boundary surface to the substrate, which normally is made of an alloy on copper basis such as CuSn4 bronze, CuNiSi or the like, the intermediate layer consisting of intermetallic compounds such as CuSn3, Cu5Sn6. The intermediate layer is harder than the contact layer and may grow as a function of temperature.
However, tin alloys or tin-alloy layers have the disadvantage that they tend to wear off due to their low hardness and the resulting low wear resistance during frequent plug-ins or due to vehicle- or engine-related vibrations, thereby causing increased oxidation—the so-called chafing corrosion. The erosion and/or chafing corrosion in turn may lead to malfunction of an electrical component of a motor vehicle assigned to the contact in question, for instance a sensor, a control unit or the like.
Another disadvantage of such tin layers is that, because of the high adhesion tendency and the plastic deformation of these contact layers, the plug forces are too high for many application cases.
Also known from practice is a contact layer of an electrical contact made on the basis of tin—also referred to as thermo-tin—, which is completely made of intermetallic phases and produced by heat treatment. Abrasion tests have shown such contact layers to be of limited use too.
In addition, AuCo alloys with a tin undercoat, silver layers with a copper undercoat or a tin undercoat or also gold layers are currently often used as contact layer in electrical contacts.
Surface or contact layers on the basis of silver, in particular, but tin as well, exhibit a cold welding tendency due to adhesion and, when combined with each other, are characterized by high friction coefficients.
Even with silver or gold layers currently used in electrical contacts, oxidative wear processes of the substrate or an intermediate layer—often made of copper or nickel and used as an adhesion layer—may occur once the layer has eroded or chipping has occurred in the layer.